changed so that invalid paramaters are not treated as hard errors, as th...

...ey can be caused by crawlers malformed links etc, and instead inform the user.
https://code.google.com/p/otwarchive/issues/detail?id=3919&thanks=3919&ts=1392660704
https://rpm.newrelic.com/accounts/115892/applications/190034/traced_errors/1005876650?original_error_id=1006005701

https://code.google.com/p/otwarchive/issues/detail?id=3933
We struggle somewhat with consistency in our messages, but we tend to put things in this way:
```flash[:error] = ts("Sorry, we couldn't find that user")``` or
```flash[:error] = ts("Sorry, could not find this user.")```
Would you change the error message to match one of these, please? :) We do tend to be specific about what couldn't be found, and say that it was the user rather than the bookmarks themselves.